Electrochemical device with high capacity and method for preparing the same

ABSTRACT

Disclosed is a method for preparing an electrochemical device, comprising the steps of: charging an electrochemical device using an electrode active material having a gas generation plateau potential in a charging period to an extent exceeding the plateau potential; and degassing the electrochemical device. An electrochemical device, which comprises an electrode active material having a gas generation plateau potential in a charging period, and is charged to an extent exceeding the plateau potential and then degassed, is also disclosed. Some electrode active materials provide high capacity but cannot be applied to a high-capacity battery due to the gas generation. This is because a battery using such electrode active materials should be charged to an extent exceeding the gas generation plateau potential in order to realize a high capacity. To solve the problems caused by the gas generation, the battery is charged to an extent exceeding the plateau potential, and then degassed. In other words, the problems including variations in outer shape of a battery and degradation in cycle life characteristics and C-rate characteristics of a battery can be solved. After the first cycle, the battery can be charged to an extent exceeding the plateau potential with no further gas generation, thereby providing a significant increase in capacity.

This application claims the benefit of the filing date of Korean PatentApplication No. 10-2005-0076488, filed on Aug. 19, 2005, in the KoreanIntellectual Property Office, the disclosure of which is incorporatedherein in its entirely by reference.

TECHNICAL FIELD

The present invention relates to an electrochemical device that utilizesan electrode active material having a gas generation plateau potentiali.e. plateau potential where gas generation occurs) in a chargingperiod. Also, the present invention relates to a method for preparingthe same electrochemical device.

BACKGROUND ART

As the mobile communication industry and the information electronicindustry have been developed markedly in recent years, lithium secondarybatteries with high capacity and low weight have been increasingly indemand. However, since mobile instruments have become moremulti-functionalized, energy consumption thereof increases, and thusbatteries used in such instruments as drive sources have been requiredto be provided with higher power and capacity. Additionally, active andintensive research and development have been conducted to substitutecobalt (Co), which is expensive and limited in supply, with inexpensivenickel (Ni), manganese (Mn), iron (Fe) or the like.

However, LiMn₂Co₄ provides a lower battery capacity when compared toLiCoO₂ by about 20% and shows a problem of Mn dissolution at highertemperature. Additionally, LiNiO₂ provides an improved energy densitywhen compared LiCoO₂, but shows a safety-related problem. Further,LiFePO₄ provides a lower capacity when compared to LiCoO₂ by about 20%and shows a problem related to C-rate characteristics.

In the case of an electrode active material having a gas generationplateau potential in a charging period, there has been the followingproblem: when a battery is charged to such an extent that gas generationdoes not occur yet, the battery shows a low capacity, and when a batteryis charged to such an extent that gas generation occurs, the batteryitself cannot be realized due to the gas generation.

DISCLOSURE OF THE INVENTION

Therefore, the present invention has been made in view of theabove-mentioned problems. The inventors of the present invention haveconducted many studies and have found that charging to an extentexceeding the gas generation plateau potential upon the first chargecycle (initial charge cycle) causes generation of a great amount of gas,however, the subsequent charge cycles cause no generation of a greatamount of gas even when they are performed to an extent exceeding theplateau potential. The present invention is based on this finding.

It is an object of the present invention to solve the problems of achange in appearance of an electrochemical device and degradation incycle life characteristics and C-rate characteristics of anelectrochemical device caused by the gas generation, by carrying out adegassing step after the electrochemical device is charged to an extentexceeding the gas generation plateau potential. It is another object ofthe present invention to provide an electrochemical device having anincreased capacity by being charged to an extent exceeding the gasgeneration plateau potential from the second charge cycle.

According to an aspect of the present invention, there is provided amethod for preparing an electrochemical device, comprising the steps of:charging an electrochemical device using an electrode active materialhaving a gas generation plateau potential in a charging period to anextent exceeding the plateau potential; and degassing theelectrochemical device.

According to another aspect of the present invention, there is providean electrochemical device comprising an electrode active material havinga gas generation plateau potential in a charging period, theelectrochemical device being charged to an extent exceeding the plateaupotential and then degassed.

Preferably, charging to an extent exceeding the plateau potential isperformed at the first charge cycle.

Hereinafter, the present invention will be explained in more detail.

The electrochemical device according to the present invention ischaracterized by the fact that it is charged to an extent exceeding theplateau potential and then degassed.

In general, some electrode active materials have a certain range ofplateau potential beyond the oxidation/reduction potentials defined byvariations in oxidation numbers of the constitutional elements of theelectrode active materials during charge/discharge cycles.

Such electrode active materials generally generate oxygen in the rangeof plateau potential. This serves to stabilize materials showinginstability caused by an increase in voltage. In other words, upon thefirst charge cycle, Li is deintercalated not by oxidation/reduction of atransition metal forming the electrode active material but by theliberation of oxygen. When oxygen is liberated, charge valance is notmade between oxygen and metals in the structure of the material, andthus Li deintercalation occurs to solve this problem. Suchdeintercalated Li may be intercalated back into a cathode while thetransition metal (e.g. Mn) forming the electrode active materialexperiences a change in its oxidation number from 4+ to 3+ upondischarge. In other words, after the aforementioned O₂ defect isgenerated (i.e. the electrode active material is activated), thecharge/discharge cycles can be accomplished via oxidation/reduction ofthe transition metal forming the electrode active material. Herein,although the transition metal (e.g. Mn), reduced from an oxidationnumber 4+ to an oxidation number of 3+, does not participate in lithium(Li) intercalation/deintercalation upon the first charge cycle, it mayparticipate in charge/discharge after the first charge cycle, therebyincreasing reversible capacity.

Therefore, the gas generation plateau potential exists at a higher levelthan the redox potential of the transition metal forming the electrodeactive material.

As described above, electrode active materials having a certain range ofplateau potential in a charging period, where gas (e.g. oxygen)generation occurs, generate a great amount of gas only in the firstcharge cycle performed to an extent exceeding the plateau potential.Thus, after the gas generated in the first charge cycle is removed, theproblem of generation of a great amount of gas can be solved even if theelectrochemical device is charged to an extent exceeding the plateaupotential during the subsequent charge/discharge cycles. Therefore,according to the present invention, when the first charge cycle isperformed to an extent exceeding the plateau potential, and then adegassing step is performed, it is possible to increase the capacity ofan electrochemical device, with no generation of a great amount of gas,by performing the second and the subsequent charge/discharge cycles toan extent exceeding the gas generation plateau potential.

Further, even when a charging voltage is reduced in the 2nd andsubsequent charging/discharging cycle after the electrochemical deviceis charged to an extent exceeding the plateau potential in the 1stcharging cycle, the electrochemical device shows a markedly increasedcapacity as compared to the same electrochemical device that is notcharged to an extent exceeding the plateau potential in the 1st chargingcycle(see FIG. 5).

In general, it is thought that the electrode active material has anactivated structure with lithium deficiency in the range of the plateaupotential, thereby generating gas in addition to an irreversible phasetransition.

When a battery using a compound represented by the following Formula 1as a cathode active material is subjected to charge/discharge cycles, aplateau appears at 4.4˜4.6V upon the first charge cycle and a greatamount of gas is generated in the plateau potential (see FIGS. 3 and 4).

In other words, in order to allow the electrode active materialrepresented by the following Formula 1 to provide high capacitycharacteristics, the battery using the electrode active material shouldbe charged to an extent exceeding the plateau potential so that theelectrode active material is activated and structurally modified.However, in this case, there is a problem of gas generation.

Such gas generation causes a change in appearance of a battery, andreduces adhesion in an electrode to adversely affect charge/dischargeuniformity of the electrode, resulting in degradation in cycle lifecharacteristics due to the generation of Li-containing byproducts.Additionally, since the gas generation causes an increase in the gapbetween electrodes, C-rate characteristics are degraded by an increasein impedance, overvoltage, or the like. In brief, such gas generationadversely affects the overall quality of a battery.

Meanwhile, if the battery is charged to an extent not exceeding theplateau potential, it is not possible to obtain high capacitycharacteristics provided by the electrode active material represented byFormula 1.

According to the present invention, the aforementioned problemsoccurring in a battery due to the gas generation can be solved, and thebattery with high capacity can be obtained by charging a battery to anextent exceeding the plateau potential at least once, and then bydegassing the battery, even if it is continuously charged to an extentexceeding the plateau potential. In other words, once the battery ischarged to an extent exceeding the plateau potential, gas generationdoes not occur any longer and the plateau disappears (see FIG. 4).

Further, as mentioned hereinbefore, the battery charged to an extentexceeding the plateau potential at least once and then degassed, showsan increased capacity even when it is subsequently charged to an extentnot exceeding the plateau potential, as compared to a battery that hasnot been charged to an extent exceeding the plateau potential (see FIG.5).

More particularly, the electrode active material includes a solidsolution represented by the following Formula 1:XLi(Li_(1/3)M_(2/3))O₂+YLiM′O₂   [Formula 1]

Wherein M is at least one element selected from the group consisting ofmetals having an oxidation number of 4+;

M′ is at least one element selected from transition metals; and

0<X<1 and 0<Y<1, with the proviso that X+Y=1.

When the electrode active material is subjected to a charge cycle at apotential level higher than the redox potential of M′, Li isdeintercalated from the electrode active material while oxygen is alsodeintercalated to correct the redox valence. In this manner, theelectrode active material shows a plateau potential.

The compound represented by Formula 1 is preferred, because it stillserves as a stable electrode active material during the subsequentcharge/discharge cycles after carrying out a charging step to a voltage(4.4˜4.8V) higher than the plateau potential and a degassing step.

Preferably, M is at least one element selected from the group consistingof Mn, Sn and Ti metals, and M′ is at least one element selected fromthe group consisting of Ni, Mn, Co and Cr metals.

When an electrochemical device using the electrode active materialcomprising the compound represented by Formula 1 is charged to an extentexceeding the plateau potential at least once and then degassed, theelectrochemical device can have a discharge capacity of 100˜280 mAh/g,preferably 170˜250 mAh/g in a voltage range of 3.0˜4.4V. When theelectrochemical device is not charged as described above, it shows adischarge capacity of approximately 90 mAh/g in the same voltage range.Therefore, the electrochemical device according to the present inventionshows a significantly increased capacity (see FIG. 5).

In addition, when the electrochemical device using the electrode activematerial comprising the compound represented by Formula 1 is charged toan extent exceeding the plateau potential at least once and thendegassed, the electrochemical device can have a discharge capacity of100˜350 mAh/g, preferably 200˜280 mAh/g in a voltage range of 3.0˜4.8V(see FIG. 4).

Hereinafter, the electrochemical device obtained by removing the gasgenerated upon the initial charge cycle performed to an extent exceedingthe gas generation plateau potential will be explained in more detail.

Preferably, the electrochemical device according to the presentinvention is a lithium ion battery.

In general, a lithium ion battery comprises a cathode having cathodeactive material slurry and a cathode collector, an anode having anodeactive material slurry and an anode collector, and a separatorinterposed between both electrodes in order to interrupt electronconduction and to perform lithium ion conduction between bothelectrodes. Also, a lithium salt-containing organic electrolyte isinjected into the void of the electrodes and the separator.

The electrode active material used in the lithium ion battery is anelectrode active material that generates a great amount of gas upon theinitial charge cycle performed to an extent exceeding the plateaupotential while generating no gas and having no plateau potential fromthe second charge cycle. For example, the electrode active material is acathode active material represented by Formula 1, which may be usedalone or in combination with at least one cathode active materialselected from the following group of cathode active materials to providea cathode: LiCoO₂, LiNiO₂, LiMnO₂, LiMn₂O4, Li(NiaCo_(b)Mn_(c))O₂(wherein 0<a<1, 0<b<1, 0<c<1, and a+b+c=1), LiNi_(1-Y)Co_(Y)O₂,LiCo_(1-y)Mn_(Y)O₂, LiNi_(1-y)Mn_(Y)O₂ (wherein 0≦Y≦1),Li(Ni_(a)Co_(b)Mn_(c))O₄(0<a<2, 0<b<2, 0<c<2, a+b+c=2),LiMn_(2-z),Ni_(z)O₄, LiMn_(2-z)Co_(z)O₄ (wherein 0<Z<2), LiCoPO₄, andLiFePO₄.

For example, the cathode can be obtained by applying a mixturecontaining the above-described cathode active material, a conductiveagent and a binder onto a cathode collector, followed by drying. Ifdesired, the mixture may further comprise fillers.

The cathode collector generally has a thickness of 3˜500 μm. There is noparticular limitation in the cathode collector, as long as it has highelectrical conductivity while not causing any chemical change in thebattery using it. Particular examples of the cathode collector that maybe used in the present invention include stainless steel, aluminum,nickel, titanium, sintered carbon, or aluminum or stainless steelsurface-treated with carbon, nickel, titanium, silver or the like. Thecollector may have fine surface roughness to increase the adhesion ofthe cathode active material thereto, and may be formed into variousshapes, including a film, sheet, foil, net, porous body, foamed body,non-woven body, or the like.

Generally, the conductive agent is added to the mixture containing thecathode active material in an amount of 1˜50 wt % based on the totalweight of the mixture. There is no particular limitation in theconductive agent, as long as it has electrical conductivity while notcausing any chemical change in the battery using it. Particular examplesof the conductive agent that may be used in the present inventioninclude: graphite such as natural graphite or artificial graphite;carbon black such as carbon black, acetylene black, ketjen black,channel black, furnace black, lamp black, thermal black, etc.;conductive fiber such as carbon fiber or metal fiber; metal powder suchas fluorocarbon, aluminum, nickel powder, etc.; conductive whisker suchas zinc oxide, potassium titanate, etc.; conductive metal oxides such astitanium oxide; and other conductive materials such as polyphenylenederivatives.

The binder helps binding between the active material and the conductiveagent or the like and binding of the active material to the collector.Generally, the binder is added to the mixture containing the cathodeactive material in an amount of 1˜50 wt % based on the total weight ofthe mixture. Particular examples of the binder that may be used in thepresent invention include polyvinylidene fluoride, polyvinyl alcohol,carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose,regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene,polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM),sulfonated EPDM, styrene butylene rubber, fluororubber, variouscopolymers, or the like.

The fillers are used optionally in order to prevent the cathode fromswelling. There is no particular limitation in the fillers, as long asthey are fibrous material while not causing any chemical change in thebattery using them. Particular examples of the fillers that may be usedin the present invention include olefin polymers such as polyethylene,polypropylene, etc.; and fibrous materials such as glass fiber, carbonfiber, etc.

The anode can be obtained by applying a mixture containing an anodeactive material onto an anode collector, followed by drying. If desired,the mixture may further comprise the additives as described above.

The anode collector generally has a thickness of 3˜500 μm. There is noparticular limitation in the anode collector, as long as it haselectrical conductivity while not causing any chemical change in thebattery using it. Particular examples of the anode collector that may beused in the present invention include copper, stainless steel, aluminum,nickel, titanium, sintered carbon, copper or stainless steelsurface-treated with carbon, nickel, titanium, silver, etc.,aluminum-cadmium alloy, or the like. Additionally, like the cathodecollector, the anode collector may have fine surface roughness toincrease the adhesion of the anode active material thereto, and may beformed into various shapes, including a film, sheet, foil, net, porousbody, foamed body, non-woven body, or the like.

Particular examples of the anode active material that may be used in thepresent invention include: carbon such as hard carbon or graphitizedcarbon; metal composite oxides such as Li_(x)Fe₂O₃(0≦x≦1),Li_(x)WO₂(0≦x≦1), Sn_(x)Me_(1-x)Me′_(Y)O_(z) (wherein Me represents Mn,Fe, Pb or Ge; Me′ represents Al, B, P, Si, a Group I, II or III elementin the Periodic Table or a halogen atom; 0<x≦1; 1≦y≦3; and 1≦z≦8);lithium metal; lithium alloy; silicon alloy; tin alloy; metal oxidessuch as SnO, SnO₂, PbO, PbO₂, Pb₂O₃, Pb₃O₄, Sb₂O₃, Sb₂O₄, Sb₂O₅, GeO,GeO₂, Bi₂O₃, Bi₂O₄, and Bi₂O₅; conductive polymers such aspolyacetylene; and Li—Co—Ni-based materials.

The separator is interposed between the cathode and the anode, andincludes a thin film having insulation property and showing high ionpermeability and mechanical strength. The separator generally has a porediameter of 0.01˜10 μm and a thickness of 5˜300 μm. Particular examplesof the separator that may be used in the present invention include:olefin polymers such as polypropylene with chemical resistance andhydrophobicity; and sheets or non-woven webs formed of glass fiber orpolyethylene. When a solid electrolyte such as a polymer electrolyte isused, the solid electrolyte may serve also as a separator.

The non-aqueous electrolyte includes a cyclic carbonate and/or linearcarbonate as an electrolyte compound. Particular examples of the cycliccarbonate include ethylene carbonate (EC), propylene carbonate (PC),gamma-butyrolactone (GBL), or the like. Preferably, the linear carbonateis selected from the group consisting of diethyl carbonate (DEC),dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and methyl propylcarbonate (MPC), but is not limited thereto. Additionally, thenon-aqueous electrolyte further comprises a lithium salt in addition tothe carbonate compound. Preferably, the lithium salt is selected fromthe group consisting of LiClO₄, LiCF₃SO₃, LiPF₆, LiBF₄, LiAsF₆ andLiN(CF₃SO₂)₂, but is not limited thereto.

The lithium ion battery according to the present invention ismanufactured by introducing a porous separator between a cathode and ananode and injecting a non-aqueous electrolyte thereto in a conventionalmanner.

The lithium ion battery according to the present invention may have anyouter shape, such as a cylindrical shape, a prismatic shape, apouch-like shape, or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a graph illustrating charge/discharge characteristics of thebattery charged according to Comparative Example 5 in a voltage range of3˜4.25V;

FIG. 2 is a graph illustrating charge/discharge characteristics of thebattery charged according to Comparative Example 6 in a voltage range of3˜4.4V;

FIG. 3 is a graph illustrating charge/discharge characteristics of thebattery charged according to Example 2 in a voltage range of 3˜4.8V;

FIG. 4 is a graph illustrating charge/discharge characteristics of thebattery charged according to Example 3 in a voltage range of 3˜4.8V uponthe second cycle; and

FIG. 5 is a graph illustrating charge/discharge characteristics of thebattery charged according to Example 4, in a voltage range of 3˜4.8Vupon the first cycle, and in a voltage range of 3˜4.4V from the secondcycle.

MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention. It is to be understood that the following examplesare illustrative only, and the scope of the present invention is notlimited thereto.

EXAMPLE 1

Cathode active material slurry was formed by usingLi(Li_(0.2)Ni_(0.2)Mn_(0.6))O₂(3/5[Li(Li_(1/3)Mn_(2/3))O₂]+2/5[LiNi_(1/2) Mn_(1/2)]O₂) as a cathodeactive material and mixing the cathode active material with carbon as aconductive agent and PVDF as a binder in a weight ratio of 88:6:6. Thecathode active material slurry was coated on Al foil having a thicknessof 15 μm to provide a cathode. Artificial graphite was used as an anodeactive material and 1M LiPF₆ solution in EC:EMC (weight ratio 1:2) wasused as an electrolyte to provide a pouch type battery.

The battery was charged to 4.8V upon the first charge cycle, anddegassed. After the battery was subjected to the second charge cycle,the battery was measured for its volumetric change.

COMPARATIVE EXAMPLE 1

The battery obtained in the same manner as described in Example 1 wascharged to 4.2V upon the first charge cycle, with no subsequentdegassing step. Then, the battery was measured for its volumetricchange.

COMPARATIVE EXAMPLE 2

The battery obtained in the same manner as described in Example 1 wascharged to 4.4V upon the first charge cycle, with no subsequentdegassing step. Then, the battery was measured for its volumetricchange.

COMPARATIVE EXAMPLE 3

The battery obtained in the same manner as described in Example 1 wascharged to 4.8V upon the first charge cycle, with no subsequentdegassing step. Then, the battery was measured for its volumetricchange.

COMPARATIVE EXAMPLE 4

The battery obtained in the same manner as described in Example 1 wascharged to 4.8V upon the first charge cycle, with no subsequentdegassing step. After the battery was subjected to the second chargecycle, the battery was measured for its volumetric change.

The following Table 1 shows the swelling degree of each of the batteriesaccording to Example 1 and Comparative Examples 1˜4, in the chargedvoltage of each battery after the first or the second cycle. TABLE 1Initial Thickness Thickness Charged thickness after charge changevoltage (mm) (mm) (mm) Ex. 1 4.8 V 4.01 4.1 0.09 Comp. Ex. 1 4.2 V 3.984.06 0.08 Comp. Ex. 2 4.4 V 3.98 4.12 0.14 Comp. Ex. 3 4.8 V 4 5.52 1.52Comp. Ex. 4 4.8 V 4.02 5.58 1.56

As can be seen from Table 1, the battery according to Example 1, whichwas initially charged to 4.8V and then degassed, and was measured forthe thickness change after the second charge cycle, shows a swellingdegree similar to that of the battery charged to about 4.2V. Thisindicates that when a battery is charged to 4.8V upon the first chargecycle and then degassed, there is no further generation of a greatamount of gas.

COMPARATIVE EXAMPLE 5

The cathode obtained in the same manner as described in Example 1 wasused, Li metal was used as an anode, and 1M LiPF₆ solution in EC:EMC(weight ratio 1:2) was used as an electrolyte to provide a coin typebattery. Charge/discharge capacity of the battery was observed in avoltage range of 3˜4.25V (see FIG. 1)

COMPARATIVE EXAMPLE 6

A battery obtained in the same manner as described in COMPARATIVEEXAMPLE 5 was measured for its charge/discharge capacity, except thatthe battery is charged to 4.4V (see FIG. 2).

EXAMPLE 2

A battery obtained in the same manner as described in ComparativeExample 5 was measured for its charge/discharge capacity, except thatthe battery is charged to 4.8V (see FIG. 3).

EXAMPLE 3

A battery obtained in the same manner as described in ComparativeExample 5 was measured for its charge/discharge capacity, except thatthe battery is charged to 4.8V upon the second cycle as well as thefirst cycle (see FIG. 4).

EXAMPLE 4

A battery obtained in the same manner as described in ComparativeExample 5 was measured for its charge/discharge capacity, except thatthe battery is charged to 4.8V upon the first cycle and to 4.4V upon thesecond cycle (see FIG. 5).

As can be seen from FIGS. 1˜4, batteries charged to a voltage exceedingthe voltage of the plateau potential according to Examples 2 and 3 showa significantly increased capacity, and the plateau potential of eachbattery disappears at the second cycle.

INDUSTRIAL APPLICABILITY

As can be seen from the foregoing, according to the present invention,it is possible to solve the problems occurring when using an electrodeactive material that provides high capacity but cannot be applied to ahigh-capacity battery. The problems relate to gas generation, since abattery using such electrode active materials should be charged to anextent exceeding the gas generation plateau potential in order torealize a high capacity. To solve the problems, the battery is chargedto an extent exceeding the plateau potential, and then degassed,according to the present invention. In other words, the presentinvention can solve the problems caused by the gas generation, includingvariations in outer shape of a battery, and degradation in cycle lifecharacteristics and C-rate characteristics of a battery. After the firstcycle, the battery can be charged to an extent exceeding the plateaupotential with no further gas generation, thereby providing asignificant increase in capacity.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment and the drawings. On the contrary, it is intendedto cover various modifications and variations within the spirit andscope of the appended claims.

1. A method for preparing an electrochemical device, comprising thesteps of: charging an electrochemical device using an electrode activematerial having a gas generation plateau potential in a charging periodto an extent exceeding the plateau potential; and degassing theelectrochemical device.
 2. The method according to claim 1, wherein theelectrode active material comprises a compound in a solid solutionstate, represented by the following Formula 1:XLi(Li_(1/3)M_(2/3))O₂+YLiM′O₂   [Formula 1]Wherein M is at least oneelement selected from the group consisting of metals having an oxidationnumber of 4+; M′ is at least one element selected from transitionmetals; and 0<X<1 and 0<Y<1, with the proviso that X+Y=1.
 3. The methodaccording to claim 2, wherein M is at least one element selected fromthe group consisting of Mn, Sn and Ti metals, and M′ is at least oneelement selected from the group consisting of Ni, Mn, Co and Cr metals.4. The method according to claim 1, wherein the electrode activematerial has a plateau potential in a range of 4.4˜4.8V.
 5. The methodaccording to claim 2, wherein the electrode active material has aplateau potential in a range of 4.4˜4.8V.
 6. The method according toclaim 3, wherein the electrode active material has a plateau potentialin a range of 4.4˜4.8V.
 7. The method according to claim 1, wherein thegas is oxygen (O₂) gas.
 8. An electrochemical device comprising anelectrode active material having a gas generation plateau potential in acharging period, the electrochemical device being charged to an extentexceeding the plateau potential and then degassed.
 9. Theelectrochemical device according to claim 8, wherein the electrodeactive material has a plateau potential in a range of 4.4˜4.8V.
 10. Theelectrochemical device according to claim 8, wherein the electrodeactive material comprises a compound in a solid solution state,represented by the following Formula 1:XLi(Li_(1/3)M_(2/3))O₂+YLiM′O₂   [Formula 1]Wherein M is at least oneelement selected from the group consisting of metals having an oxidationnumber of 4+; M′ is at least one element selected from transitionmetals; and 0<X<1 and 0<Y<1, with the proviso that X+Y=1.
 11. Theelectrochemical device according to claim 10, wherein M is at least oneelement selected from the group consisting of Mn, Sn and Ti metals, andM′ is at least one element selected from the group consisting of Ni, Mn,Co and Cr metals.
 12. The electrochemical device according to claim 10,which shows a discharge capacity of 100˜280 mAh/g in a voltage range of3.0˜4.4V, after being charged to an extent exceeding the plateaupotential and then degassed.
 13. The electrochemical device according toclaim 10, which shows a discharge capacity of 100˜350 mAh/g in a voltagerange of 3.0˜4.8V, after being charged to an extent exceeding theplateau potential and then degassed.
 14. The electrochemical deviceaccording to claim 8, which is designed to be used after being chargedto an extent exceeding the gas generation plateau potential.
 15. Theelectrochemical device according to claim 8, which is designed to beused after being charged to an extent not exceeding the gas generationplateau potential.
 16. The electrochemical device according to claim 8,which is a lithium secondary battery.
 17. The electrochemical deviceaccording to claim 10, which is a lithium secondary battery.
 18. Theelectrochemical device according to claim 11, which is a lithiumsecondary battery.
 19. The electrochemical device according to claim 12,which is a lithium secondary battery.
 20. The electrochemical deviceaccording to claim 13, which is a lithium secondary battery.